1. Field of the Invention
The present invention relates to a method of manufacturing a lithographic printing plate, and in particular, to a method of manufacturing a lithographic printing plate which is suited to the manufacturing of digital direct printing plates which are applied to direct plate-making systems.
2. Description of the Related Art
A so-called conventional printing plate is widely used as a conventional photosensitive lithographic printing plate. After this conventional printing plate is mask-exposed (planarly exposed) via a lith film, the non-image portions of the photosensitive layer are dissolved and removed, such that the printing plate carries a desired image.
In recent years, digitizing techniques for electronically processing image information by using computers have come into wider use, and new plate-making techniques corresponding to these digitizing techniques have come to be put into practice. Specifically, light having high directivity, such as laser light, is modulated in accordance with digitized image information. By scan-exposing an original plate of a lithographic printing plate by using this laser light, a computer-to-plate (CTP) technique can be realized in which a printing plate is manufactured directly without use of a lith film. The demand for so-called digital direct plates which are suitable for this CTP technique continues to increase.
It is known that the quality of such a digital direct printing plate (i.e., the stability of the sensitivity, the scratch resistance, the ability to withstand repeated printing, and the like) is more easily affected, than the quality of a conventional printing plate is, by the drying conditions or the heating hardening conditions (curing conditions) of the photosensitive coated layer formed on the support. In particular, it is known that quality is affected by the heating conditions (temperature and time) during curing in which the support and the photosensitive coated layer are heated in order to accelerate the hardening of the photosensitive coated layer after evaporating and drying have been carried out until the organic solvent of the photosensitive coated layer is contained in a predetermined amount. Specifically, for example, when a plurality of types of digital direct printing plates, which differ only in that the thicknesses of their respective supports (which are formed by aluminum plates or the like) differ, are manufactured, if these supports and photosensitive coated layers are heated under the same conditions, differences in quality of the supports of the digital direct printing plates, which differences in quality are of an extent that cause problems in practice, arise due to the differences in the heat capacities of the supports.
Further, in conventional lithographic printing plate manufacturing processes, the drying process and the curing process for the support and the photosensitive coated layer are not clearly separated. For example, as disclosed in Japanese Patent Application Publication (JP-B) No. 6-24673, while a support on which a photosensitive coated layer is formed is conveyed in one direction, drying and curing are carried out by a hot-air drying system drying device which blows out hot air into a heating furnace disposed along the conveying path of the support. However, in this hot-air drying system drying device, it is difficult to change the heating conditions within a short period of time. Accordingly, when the thickness or the width or the like of the support is changed, in order to change the heating conditions for the support and the photosensitive coated layer, in most cases, the conveying speed of the support, i.e., the speed of manufacturing the lithographic printing plate, must be changed, and it is difficult to stabilize the production speed of digital direct printing plates.
Further, it has recently become clear that, in order to make the quality (i.e., the stability of the sensitivity, the scratch resistance, the ability to withstand repeated printing, and the like) of digital direct printing plates sufficiently high and stable, there is the need to supply a great amount of heat to the photosensitive coated layer at the time of curing, as compared with conventional printing plates. (With regard to this point, refer to, for example, Japanese Patent Application No. 11-301240, the applicant of which is the same as the applicant of the present application.) Thus, when digital direct printing plates and conventional printing plates are to be manufactured by using the same equipment, the manufacturing speeds are controlled by the heating capacities with respect to the supports and the photosensitive coated layers, and the speed of manufacturing the digital direct printing plate must be decreased greatly as compared to that of the conventional printing plate.
As a means for overcoming the above-described insufficient heating capacity, JP-B No. 6-49175 discloses a method of supplying a large amount of heat to a support in a short time by heating rollers which contact the reverse surface of the support. Further, Japanese Patent Application Laid-Open (JP-A) No. 2-227160 discloses a method of controlling an amount of heat supplied to a support by changing the time over which heating rollers contact a support. However, when a support is heated by these methods, minute scratches are formed in the reverse surface of the support by the heating rollers due to the support expanding or contracting due to the temperature difference between the support and the heating rollers, or due to the linear speed of the heating rollers and the conveying speed of the support not being equal or the like. These minute scratches do not present problems in practice in conventional printing plates. However, with digital direct printing plates, there are cases in which the photosensitive coated layer is scratched and problems arise with respect to quality, at the time that the support is wound up in a coil form or at the time that plural printing plates are stacked.
Thus, there is the demand for a technique for manufacturing high quality digital direct printing plates stably and at a low cost, in which the conditions for the heating of the support and the photosensitive coated layer during a drying and heating process can be changed quickly within a wide range, and heating can be carried out without the support and the photosensitive coated layer being contacted, without greatly remodeling the equipment for manufacturing conventional printing plates.